feat(shortcuts): chord-drag window move, a zen-mode key, and a don't-fire-while-typing guard

[100mountains]

Part of the final derp-fork contribution batch (#2569). Three related pieces on one chord family — Cmd+Alt on macOS, Ctrl+Alt everywhere else (meta on Windows is the Windows key, which nobody wants in a chord):

• window-drag (new self-contained feature dir): hold the chord and drag anywhere — toolbars, tab strip, or web content — to move the window. Most useful in zen mode and multirow tabs, where the draggable titlebar is hidden or crowded. Bare-Cmd drags are deliberately not used: Cmd+click opens links and toggles multi-selection, and web apps (Figma, Miro, maps) use Cmd+drag for canvas panning. The chord is re-verified on every mousemove, so a mouseup lost outside the window can never leave the window glued to the pointer.
• A default shortcut: chord+Z toggles zen mode via the existing floorp-toggle-zen-mode action. Saved configs are merged over defaults so the new entry reaches existing profiles (caveat noted in-code: a default the user deleted comes back until configs carry a schema version — deleted-vs-rebound isn't distinguishable today).
• Typing guard in the shortcut controller: bare printable single-key shortcuts no longer fire while typing — input/textarea/contenteditable, the shadow-DOM urlbar/searchbar/findbar hosts, and remote content via browser.isInputFocused. Chorded shortcuts are unaffected.

Validation honesty: daily-driven in derp on macOS (FF152 runtime); the guard's remote-content branch matches how derp shipped it. Not exercised on Windows/Linux beyond the chord choice logic.

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