feat(app): rtl header for Hebrew and Arabic systems - #48856

Issue for this PR

N/A — found while running desktop dev on Hebrew Windows (no existing issue).

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

On Hebrew/Arabic Windows the native caption buttons (minimize/maximize/close) sit on the left side of the window, but the desktop titlebar always reserved space on the right (margin-right: auto) and never switched to RTL, so the header overlapped the caption controls.

Changes:

• systemPrefersRtl() in packages/app/src/i18n/desktop-native.ts detects RTL OS languages from navigator.languages (which is the OS language in Electron), including Hebrew (he plus legacy iw). Hebrew has no UI bundle, so it keeps English strings but now gets RTL chrome.
• New language.systemDirection() in the language context, consumed only by the titlebar: <header dir> plus margin-left: auto under RTL system languages / margin-right: auto under LTR ones. Document direction and conversation content still follow the UI locale — only the header follows the OS language, because only the native window chrome depends on it.
• New direction.toggle command (mod+shift+d, View category) that flips the content direction, same as the debug-bar DIR switch. command.direction.toggle was added with English fallback text in the locale files; real translations should go through translate:app.

The second commit (fix: restore triple-slash references in custom-elements) is a prerequisite, not an unrelated feature: bun typecheck fails identically on clean dev without it, so no green CI is possible without it. Happy to split it into its own PR on request.

How did you verify your code works?

• bun test src/i18n/desktop-native.test.ts (new systemPrefersRtl cases: he-IL, iw-IL, ar-EG, LTR negatives, tag normalization) plus the command/layout suites: 27 pass, 0 fail.
• Full monorepo bun typecheck: 30/30 successful.
• Ran electron-vite dev on Hebrew Windows and watched the header mirror with margin-left: auto; toggled content direction with the new hotkey.

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR