feat(read-only-viewer): editor viewer mode + kicadSetReadOnly binding

Anonymous / non-member sessions open PUBLIC projects as read-only viewers.

- kicadSetReadOnly(bool) embind (merged kicad_editor + pcbnew/eeschema/
  pl_editor TUs): sets the PCBJAM_READ_ONLY flag + Prj().SetReadOnly (greys
  the setup dialogs). Polls until the frame exists.
- read-only-mode.ts: resolveReadOnly(access, win) — server `access:"read"` or
  ?readonly=1 (narrow-only; no ?readonly=0). ToolPage threads it in, omits
  saveBytes (MEMFS-only saves).
- WasmTool: chrome force-hidden with a "View only" pill (toggle + Cmd+\
  disabled), presence/cross-app/comments/drift skipped, save-driven room
  writers unregistered, wasm frame locked via kicadSetReadOnly failing CLOSED
  (stale bundle → boot error, never a writable frame).
- collab: bindKicadCollab {readOnly} — inert DOWN hook, never seeds a room;
  UP observer + adopt stay live so peer edits render. index.ts / sheet-manager
  thread readOnly + drop initial awareness (invisible observer).
- Reference backend emits access:"write".

Bumps kicad + web/pcbjam-shared to the read-only-viewer commits.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012DN9py5GuPdExaaFzE4k27

web/backend/src/server.ts

@@ -190,6 +190,8 @@ async function main(): Promise<void> {
         body: {
           project: await project(params.scope),
           files: await walk(PROJECT_DIR),
+          // No auth here: every caller may write (explicit contract conformance).
+          access: "write" as const,
         },
       };
     },