Add "Run at startup" toggle in Settings

[trsdn]

Closes #15.

Summary

Adds an in-app Run at startup toggle (Settings → Startup) that works across every distribution channel — Inno installer, portable ZIP, and MSIX/Store — replacing the old install-time-only autostart that could never be changed from inside the app.

What changed

New module src/autostart.rs — an AutostartBackend trait (mirroring the existing CredentialStore pattern) with two mutually-exclusive Windows backends, plus enable_verified/disable_verified write-then-read-back helpers and a backend() factory that picks packaged vs unpackaged at runtime.

• Unpackaged (installer + portable): reads/writes/deletes the HKCU\…\CurrentVersion\Run value OpenWritr (quoted current_exe, no args — an argless start is already silent tray mode). Includes:
  • stale-path repair — rewrites the value on read when the exe has moved (portable users relocating the folder);
  • OS-disabled detection by parsing the 12-byte StartupApproved\Run blob (first byte ≠ 2 ⇒ disabled) → surfaced as DisabledByOs; never re-enabled on launch;
  • one-time migration of the legacy {userstartup}\OpenWritr.lnk shortcut into the registry value.
• MSIX/Store: WinRT StartupTask OpenWritrStartup (RequestEnableAsync/Disable), mapping DisabledByUser/DisabledByPolicy to DisabledByOs with a reason string.

UI (src/settings_ui.rs) — new "Startup" section with checkbox “Start OpenWritr automatically when I sign in to Windows” and an info button noting it starts silently in the tray. The toggle is deliberately not a Settings serde field: it's OS state, writes through immediately, is verified by read-back, never touches persist()/dirty/SettingsRevision, and re-reads real OS state each time the window opens (so external Task Manager / Settings → Apps → Startup changes show truthfully). When DisabledByOs the checkbox is disabled with an explanation; write failures render through inline_error (no silent no-op).

Installer — both installer/openwritr.iss and installer/openwritr-x64.iss drop the {userstartup} shortcut and instead write the same HKCU Run value (gated on the autostart task), and always remove the value on uninstall even if enabled later from the app. AppxManifest.template.xml stays Enabled="false" (the in-app toggle calls RequestEnableAsync).

Deletion of the legacy {userstartup}\OpenWritr.lnk shortcut is coupled to the autostart task ([InstallDelete] … Tasks: autostart): the shortcut is only removed when the replacement Run value is actually written in its place. If an existing user deselects the autostart task on upgrade (or in any silent/default run where the task is unset), the legacy shortcut is left intact so the app-side migration in autostart.rs can still pick it up — a previously active autostart is never silently dropped. An InitializeWizard [Code] hook additionally preselects the autostart task when a legacy shortcut or an existing Run value is detected (Inno otherwise remembers a prior deselection across upgrades).

Diagnostics — the diagnostics bundle (autostart-status.json) and --self-check now report the resolved backend and current state.

Docs — README installer + portable sections updated.

Validation

• cargo fmt --check — clean (exit 0).
• cargo clippy --all-targets — no new warnings from the changed files (pre-existing warnings only).
• cargo test --bins — test result: ok. 283 passed; 0 failed; 8 ignored (main bin suite), plus the other bin suites green. Includes new unit tests: enable/disable/read round-trip, stale-path repair, StartupApproved disabled-blob parsing, legacy shortcut migration, and a settings_ui test asserting a write failure surfaces an error instead of silently flipping state.
• Inno scripts — both openwritr.iss and openwritr-x64.iss compile with ISCC.exe at exit 0 (verified against a staged scratch tree).

Note: the .iss scripts are not covered by cargo test; an ISCC compile check in CI would close that gap, but that's left as a separate follow-up (out of scope for this PR).

Could NOT be verified automatically — please check manually

• Real sign-out / sign-in autostart on installer and portable builds (silent tray start).
• Disabling the entry in Task Manager → Startup showing up in-app as DisabledByOs.
• The MSIX StartupTask path end-to-end (CI build here was arm64 desktop, not packaged).
• Uninstall leaving no Run value and no leftover shortcut, including the enabled-later-in-app case.
• Legacy .lnk → registry migration and the new task-preselect / gated-deletion behaviour on a machine upgrading from a prior install.