feat: add persistent per-note read-only mode

[t0ma5]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

This continues @vbernier02's idea and first implementation in #123 (feat: read only mode, Fixes #69). He handed the work to @naveensingh when he got busy. This PR is a new branch from current main so we do not push to his fork. It is not a claim that the feature is original.

• Overflow actions Make read-only / Allow editing (no extra toolbar icons)
• Persist is_read_only on the note (Room v5). Review on feat: read only mode #123 first asked for session-only; this keeps persistence so the mode survives restart, matching the [fr] read only #69 request to avoid accidental overwrites. Happy to switch if you still want session-only.
• Text notes: typing blocked, spell-check/suggestions off, keyboard hidden, selection/copy still work, no keyListener = null (that caused the long-press NPE on feat: read only mode #123)
• Undo/redo hidden on startup and while read-only
• Checklists: checks, add FAB, reorder, and mutation menus blocked

Tests performed

• compileFossReleaseKotlin / KSP on current main

Closes the following issue(s)

• Fixes [fr] read only #69
• Continues feat: read only mode #123

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• I understand every change in this pull request.

[fossifybot]

Fossify accepts code contributions only for open issues labeled help wanted. This pull request does not meet that requirement or one of the documented exceptions, so it is being closed without review. Please read the contribution guidelines before starting work.