[turinglambdaai]

Problem

The MSI finish page offers Launch Taskly. On selecting it, the WinUI 3 window can appear in the default system-light/English presentation before the Racket backend has started and returned persisted preferences. This produces a jarring white flash for users who configured Taskly's dark theme, especially noticeable immediately after installation. The root Grid also lacks an explicit theme-aware background, leaving transparent areas to the window's default fill.

Changes

• Read only language and theme from ~/.taskly/config.ini before Window.Activate(), respecting the Windows USERPROFILE → HOME precedence, comments, whitespace, case-insensitive keys, last-key-wins and documented fallbacks.
• Apply the initial language and theme as the window is constructed. The existing asynchronous get_settings RPC remains authoritative and still refreshes the UI after backend startup.
• Give RootGrid a ThemeResource TasklySurfaceBrush background so the full client area paints consistently in dark and light mode.
• Update product/data-format contracts for the startup appearance behavior.

Verification

• Changes isolated to Windows host and documentation; no generated Rivet clients, MSI tooling, or Racket contracts modified.
• GitHub branch diff reviewed against main.
• Windows CI build (expected to run on this PR).
• Manual Windows 10/11 QA:
  1. Set theme Dark while OS is Light, quit and relaunch Taskly from the MSI Launch Taskly checkbox; confirm no white/English first frame.
  2. Repeat with light/system themes and both language settings; check title bar and content agree.
  3. Launch with no config, malformed values, comments, duplicate keys, and HOME fallback; verify defaults/reconciliation.

Scope note: This removes Taskly's launch-time white flash; the stock WiX MSI wizard's native white checkbox/license controls belong to Rivet's upstream installer UI and are not restyled by this PR. A separate upstream change would be needed if those controls themselves are the white areas being reported.

Does not merge automatically.