Mobile polish pass — graph, editor, header on small viewports

[unforced]

Why

Lens renders on mobile but wasn't designed for it. Several rough edges bite on phone:

• Full-vault graph is nearly unusable (zoom/pan fighting the page scroll)
• Note editor's side-by-side preview doesn't fit; needs a toggle or stacked layout
• Header gets crowded with vault switcher + theme toggle + sync indicator + install prompt
• /capture works well on mobile actually — keep that, don't regress

Scope

• Header: collapse on narrow widths to a hamburger menu; top bar keeps the most essential control (sync status dot) always visible
• Editor (/new, /notes/:id/edit): stacked "edit | preview" tabs on narrow widths instead of side-by-side split
• Full-vault graph: add a "fit to screen" button; prevent document scroll when touching inside the canvas; increase node hit target
• Neighborhood graph: already small enough, mostly fine — confirm it doesn't cause layout jumps
• List views: the existing filter row wraps awkwardly on narrow screens — audit and tighten
• Safe-area insets: respect env(safe-area-inset-*) on installed iOS PWAs so content doesn't hide under the home indicator / notch

Testing

• Verify on actual iPhone Safari (via Tailscale HTTPS URL)
• Verify on iPhone installed PWA (same URL, Add to Home Screen)
• Verify on Android Chrome
• Check landscape + portrait
• Chrome DevTools mobile emulation as a fast first-pass

Out of scope

• Native-feel gestures (swipe to archive, etc.) — those are a separate deeper arc
• Mobile-only features (pull-to-refresh) — case by case, probably not universally wanted
• Redesigning anything — this is "make existing features work well on phone," not a UX rewrite

Priority

Low-mid. Do after the content-oriented v0.3 work lands, since each new feature will need its own mobile review anyway.