fix(sync): prevent whitespace-only board names from blocking sync

[RodriSanchez1]

Fixes the sync-blocking half of #2281.

Problem

A board whose name is whitespace-only (" ") is accepted everywhere on the way out and rejected at the last step:

1. TileEditor.component.js:532 — disableSubmit={!currentLabel} correctly blocks '', but ' ' is truthy, so Save is enabled.
2. handleLabelChange clears labelKey, so the folder board is created with name: ' ', nameKey: ''.
3. extractBoardName returns board.name unchanged, because ' ' is truthy.
4. Swagger (swagger.yaml:1522-1532) declares name as required / type: string with no minLength — required means the key is present, not non-empty — so it passes.
5. Mongoose (Board.js:8-12) has trim: true, and the trim setter runs before the required validator. ' ' becomes '' and the save fails.

Result: 409 from createBoard, 500 from updateBoard, and because UPDATE_BOARD sets PENDING before the call and the catch only logs, the board is re-pushed every sync cycle forever.

author fails identically — transformBoardForUser used author: userName || userEmail, and a display name of ' ' is truthy, survives the ||, and dies at the same trim + required pair.

Changes

• TileEditor.component.js — disableSubmit={!currentLabel || !currentLabel.trim()}. Covers both the create (Board.container.js:588) and edit (:1172) paths, since both read tile.label behind this button. Deliberately not trimming in handleLabelChange, which would fight the user mid-typing.
• Board.utils.js — extractBoardName treats whitespace-only name and nameKey segments as missing and falls back to DEFAULT_BOARD_NAME; transformBoardForUser trims userName before the ||.
• Board.constants.js — DEFAULT_BOARD_NAME = 'Untitled Board', the same string used before 78866f6. A plain constant rather than an i18n message because extractBoardName has no intl in scope; localizing it is a signature change worth doing separately.
• Board.actions.js — pushBoard exceptions now carry status and apiError. The API already returns the exact Mongoose message (board.js:38, board.js:262) naming the failing field, but the raw axios rejection's .message is only "Request failed with status code 500", so that detail was being discarded at the client boundary.

Scope — what this does not do

Existing affected boards are not repaired. The extractBoardName fallback only runs via transformBoardForUser, which applies to default/no-email boards. A user-owned board already in localStorage with a whitespace name goes straight to updateApiBoard and will still fail on every cycle. The fix that would cover it — trimming at the wire in API.createBoard / API.updateBoard — was intentionally deferred, and is tracked as an open item on #2281.

The retry loop is untouched. Any permanently-rejected board is still re-pushed indefinitely. That needs an attempt counter in syncMeta and interacts with the graduation logic in classifyBoardsForPush, so it warrants its own change.

One board remains unexplained. The originally-reported board reads people in the DB, which derives from nameKey: cboard.symbol.people and is non-blank under every path examined. No code path was found that blanks it. The telemetry change should settle whether it is failing on name or author on the next sync cycle from an affected user — so please don't read a drop in pushBoard exceptions as proof the diagnosis was complete.

Also worth noting: commit 78866f6 (return '' in extractBoardName) is not the regression. That branch needs a board with neither name nor nameKey; all 44 default boards in boards.json carry a nameKey deriving a non-blank name, and the tile path always supplies a truthy name. It was a latent hazard, now closed by the fallback, but the live bug is whitespace.

Testing

eslint clean on all four files. No existing test asserts on disableSubmit, on trackSyncException's pushBoard properties, or on extractBoardName / transformBoardForUser — those two helpers have no direct coverage today, which is a gap worth filling.

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