Fix #126: stop note tiles from triggering ENOENT on undefined file path

[Hude06]

Fixes #126.

Root cause

Tracing the ENOENT from the issue:

1. canvas-rpc.js:104 — RPC tileCreate defaults tileType to "note". For a note with no backing file, it falls through to createFileTile("note", x, y, undefined).

2. tile-manager.js:626-628 — Builds the viewer webview URL with \tilePath=${encodeURIComponent(filePath)}`. encodeURIComponent(undefined)returns the literal string"undefined", so the URL becomes viewer.html?tilePath=undefined&tileMode=note`.

3. viewer/src/App.tsx:108-114 — Parses tilePath from the URL. params.get("tilePath") returns the string "undefined". The if (tp) guard treats it as truthy and calls setSelectedPath("undefined").

4. viewer/src/App.tsx:324 — Subsequently calls window.api.readFile("undefined"). The desktop bridge resolves the relative path against the app install dir, producing the path from the issue:

   ENOENT: no such file or directory, open 'C:\Users\…\AppData\Local\Programs\Collaborator\undefined'
   

Fix

Two surgical changes, both upstream of the IPC call:

collab-electron/src/windows/shell/src/tile-manager.js

Build the viewer URL with URLSearchParams and only set tilePath when filePath is a non-empty string. Note tiles without a backing file now get a viewer URL with no tilePath at all, so the viewer's existing if (!selectedPath) return; guard catches them naturally and never calls readFile.

collab-electron/src/windows/viewer/src/App.tsx

Defense in depth — treat the literal strings "undefined" and "null" as missing tilePath. This protects against any other caller that might still emit a stringified nullish value (e.g. older serialized state, third-party RPC callers).

What this does not do

This PR fixes the crash but does not implement in-memory note content. Note tiles created without a backing file will render an empty viewer rather than throwing. The viewer already accepts a tileMode query param but does not currently branch on it — adding real in-memory note rendering is a larger scope and belongs in a follow-up.

Verification

Repro from the issue:

1. Create a note tile via the canvas tools.
2. Before this PR: ENOENT error in the main process log, viewer fails to render.
3. After this PR: viewer renders an empty tile, no error in the log.

[yiliush]

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[Hude06]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.