Add mobile view-only support

Below md width, the app now forces view-only (same canEdit mechanism as a
shared view-only diagram) rather than trying to support touch editing:
palette and inspector panels hide, editing controls disable, and the top
bar collapses to just Diagrams + Account. A bottom tab bar swaps between a
full-screen canvas and a full-screen Bill of Materials, since there's no
room for both side by side. Canvas gets a few mobile-specific trims too:
MiniMap and the interactivity lock button hide whenever canEdit is false
(the lock button doesn't actually grant editing either way — nodesDraggable/
nodesConnectable already override it — so leaving it visible would just be
misleading), and h-screen is replaced with h-dvh throughout so the layout
doesn't get clipped by a mobile browser's collapsing address bar.

Also fixes quick-add's anti-stack offset (24px — far smaller than a device
node) to actually tile devices apart instead of leaving them nearly
overlapping, and logs a deferred "Auto-arrange" layout feature in
organized-ideas.md, surfaced by mobile visitors having no way to fix an
overlapping diagram themselves.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017DUU6CnxECCDeqDNYJgr5x

organized-ideas.md

@@ -82,6 +82,7 @@ Stated explicitly during planning: this needs to run **entirely locally** during
 - **Recommended:** whatever visual grouping happens in compact view must be purely a canvas rendering concern — the BOM math (cable counts, lengths, totals) always operates on the real underlying connections, never on the aggregated display. Otherwise compact view could silently produce a wrong shopping list.
 - **Implementation detail, still loose:** exactly what "connection-type group" means for aggregation purposes isn't fully pinned down — grouped by cable type, by port family, by direction, some combination? Low-stakes to leave open since this is frontend-only and easy to iterate on visually once it's being built.
 - **Decided: toggle scope.** Per-device (each device node expands/collapses independently), defaulting to **compact** for a newly-placed device — consistent with the palette categories already defaulting to collapsed. Plus **"Expand all" / "Collapse all"** buttons on the canvas for quickly toggling every device at once, rather than clicking through each one individually.
+- **Future TODO, not yet built: an "Auto-arrange" layout action.** Surfaced while building mobile view-only support (2026-09-28) — nothing stops two devices' saved positions from overlapping (only quick-add's own placement got a bigger anti-stack grid), and expanding a device grows its node size, which can turn a fine compact layout into an overlapping expanded one. On desktop this was always self-correctable (whoever's looking has edit access and can just drag nodes apart), but a mobile/view-only visitor now has no way to fix it themselves — they're stuck with whatever layout the diagram's owner left. Needs a real graph-layout pass (something like `dagre`), a decision on when it runs (probably a manual button rather than automatic-on-load, so a diagram doesn't silently rearrange itself), and to account for compact vs. expanded node sizing. Deliberately deferred rather than built speculatively — it's a standalone feature, not something to fold into the mobile branch.
 
 ## 5. Device Menus