RustBox 0.2.15 release manifest

0.2.15 release changes and retained feature set:
- fixes HTML signature formatting so toolbar actions preserve the selected text and reliably toggle formatting
- gives new messages with visible signatures a real editable body entry above the signature and softens/spaces the composer-only separator
- strips the editor-only body-entry marker from serialized outgoing HTML while retaining signature de-duplication metadata
- defaults wide message panes to compact single-line rows, with an Appearance setting to disable the behavior
- adds per-identity signature insertion mode: silent append-on-send (default) or visible/editable in the composer
- adds a durable SQL blob-deletion outbox used by filesystem/shared POSIX/NFS and S3 backends; attachment/draft cleanup is transactionally queued and retried until successful
- failed finalized uploads and failed destination objects from storage migration are also queued for cleanup; storage health probes reuse a fixed key to avoid unbounded probe-object buildup
- suppresses realtime “new message” toast/system notifications for Drafts saves while preserving normal folder/list refresh and `\Draft` + `\Seen` IMAP flags
- adds `Delete draft` in the composer; IMAP deletion is UID/UIDVALIDITY-checked and local draft attachment rows/blobs are cascaded after the server draft is removed
- adds an explicit `Save draft` action before `Cancel`; it persists the current message and attachments as a real IMAP Drafts snapshot without changing background autosave behavior
- explicit IMAP draft saves surface snapshot/APPEND failures instead of reporting a false success
- draft recovery restores attachment metadata from browser snapshots plus the authoritative server draft
- attachment chips show all uploaded files, including inline images/links, and new uploads merge by unique attachment ID instead of replacing visible entries
- fixes multi-attachment/inline-image insertion so new items append instead of replacing the selected image/content
- uses DOM Range insertion with stable caret placement for embedded images and CID file links
- floats image sizing/alignment actions over the editor instead of consuming composer height
- supports Ctrl/Cmd+V image embedding, file drop at the caret, dragging embedded images within the body, and four resize handles
- labels embedded images separately from regular/image attachment chips
- keeps attachment re-embed actions compact and allows separate same-name/same-size files
- compact recipient address summaries with expandable full recipient lists
- adds default pagination at 300 messages per page, plus optional smooth progressive loading and full-folder loading
- adds a composer `Hide fields` / `Show fields` control so From/To/Cc/Bcc/Subject/options can collapse and the editor can use the freed vertical space
- inline-by-default image attachments, re-embedding, CID file links, and inline image layout controls
- opens message-body links without an explicit target in a new tab/window instead of navigating inside the sandboxed mail-preview iframe; explicit targets remain intact
- shows the login success toast only after a real POST login; session restoration/page refresh is silent
- improves non-ASCII/Polish mail display with explicit UTF-8 preview documents and broader RFC 2047 header decoding (including ISO-8859-2/Windows-1250)
- previews PDFs from the authenticated inline attachment endpoint instead of a blob iframe for Safari/iPhone compatibility
- presents compose attachment/message size limits as compact raw values without implementation-detail descriptions
- retains the 0.2.9 realtime notification, shortcut and 300-message mailbox behavior
- adds opt-in adaptive HTML-mail colors in Appearance and folds both it and mailbox loading mode into the unreleased V050 runtime migration; V051 remains next
Architecture:
- database_backend = sqlite | mysql | pgsql selects authoritative SQL only
- blob storage and search are independent from the SQL engine
- PostgreSQL/MySQL single-node deployments may continue to use data_dir-backed filesystem/Tantivy
- cluster mode clears an implicit/default data_dir; explicit data_dir is not forbidden, but local-only capabilities are rejected

Blob storage:
- filesystem: local single-node or shared POSIX with storage.shared=true
- shared POSIX examples: NFS, CephFS, GlusterFS
- s3: AWS S3 and S3-compatible endpoints such as MinIO/Ceph RGW
- attachment rows use logical storage keys and SHA-256 metadata
- storage migration command: rustbox storage migrate --from filesystem:/path --to configured [--dry-run] [--verify]

Search:
- local_tantivy: single-node only
- opensearch: cluster-capable external index
- disabled: no full-text index
- OpenSearch operations are tenant-scoped by user_hash
- durable SQL outbox provides claim/lease, retry/backoff and dead-letter handling
- rebuild command: rustbox search reindex

Database/schema:
- migrations/{sqlite,mysql,pgsql}/schema.sql contain the current fresh schema
- compatibility baseline is V049; former V047-V049 changes are consolidated into `schema.sql`, while V050 remains the single runtime migration
- databases older than V049 must be upgraded to schema V049 before using this build
- V050 contains `mail_preferences.compact_wide_rows`, `identities.signature_insert_mode`, `display_preferences.adaptive_email_colors` and `mail_preferences.message_loading_mode` because this release has not shipped yet
- next migration number: V051

Docker examples:
- docker/compose-single-sqlite.yml
- docker/compose-ha-garage.yml (2x RustBox + Envoy + Valkey + PostgreSQL + OpenSearch + Garage S3)
- docker/compose-postgres-s3-opensearch.yml (PostgreSQL + SILO/S3 + OpenSearch)
- docker/compose-mysql-nfs-opensearch.yml (MySQL + real NFS volume + OpenSearch)

Operations/docs:
- standalone `scripts/migrate_roundcube.py` imports Roundcube contacts/identities using RustBox config.toml and supports SQLite/MySQL/PostgreSQL targets
- README.md documents 0.2.15 configuration
- docker/README.md documents Compose variants
- docs/STORAGE_SEARCH_OPERATIONS.md documents failure modes, shared-storage requirements and migration behavior
- TODO.md leaves live SILO/MinIO/AWS/Ceph and real multi-node acceptance validation explicitly pending

Build/release metadata:
- `scripts/package-release.sh` runs the full Rust release gate before creating ZIP/SHA256 artifacts
- Cargo.toml version = 0.2.15
- rust-toolchain.toml pins Rust 1.99.1
- Cargo.lock is retained for reproducible application builds; the release gate generates it only when missing
- CI and scripts/build-release.sh use the retained Cargo.lock (generating it only if missing), then run locked tests/Clippy/release build