refactor: give duplicated helpers one home

[vibhavkatre]

Second of three cleanup passes (backlog #9). Stacked on #24 — review that one first; this PR's base is refactor/dead-code-frontend, so the diff shown here is only the dedupe work.

Seven helpers existed in two or three copies each. Every copy was byte-identical or trivially equivalent, so each is now defined once and imported.

Helper	Copies	New home	Note
distanceToSegment	3	diagram/geometry.js	byte-identical copies in whiteboardModel, strokeSimplify, useFlowchartInteraction
clone	3	utils/clone.js	useDiagramStore, history, useClipboard
isEditingText	2	utils/dom.js	useKeyboard, useCanvasPaste
wrapLineCount	2	diagram/textMetrics.js	same algorithm, two signatures
pointsToPath	2	diagram/svgPath.js	useThumbnail imported both under aliases
localStorage JSON read/write	3	utils/localStore.js	useRecentColors, useRecentShapes, useAppSettings
minimap navigator (~50 lines)	2	composables/useMinimapNavigator.js	Minimap.vue, WhiteboardMinimap.vue

Why these particular merges matter

• distanceToSegment — every "how far is the cursor from this polyline?" test (stroke simplification, eraser/line hit-testing, flowchart edge picking) now measures through the same function instead of three copies that could drift.
• clone — the JSON round-trip semantics are now written down. Dropping undefined keys is what keeps a history snapshot equal to what a save persists, so this must not be casually "upgraded" to structuredClone (which keeps them, and throws on the functions a reactive proxy can carry).
• wrapLineCount — mind-map and flowchart node sizing agreed by coincidence before; now by construction. The shared version takes perLine, and the mind map derives that via the new charsPerLine().
• minimap navigator — the two navigators duplicated fit-to-box scaling, the viewport rect and click/drag panning, so they could disagree about the same gesture. They differ in exactly one respect, which the composable now models explicitly: Minimap crops the viewport rect to its box (clampedViewRect), WhiteboardMinimap does not (viewRect). Also drops a vestigial element ref in WhiteboardMinimap that nothing read.

One intentional behaviour fix

useAppSettings wrote settings with a bare localStorage.setItem while the other two call sites guarded theirs. In Safari private mode setItem throws, so toggling dark mode threw inside a watcher there. writeJson guards uniformly.

Testing

• 127 vitest pass (102 before — 25 new tests across textMetrics, svgPath, geometry, clone, dom, localStore)
• New tests deliberately cover the merged edge cases: long-word wrapping, the "finish the current line before a long word" branch, segment-distance clamping past endpoints, and the localStorage corrupt-JSON / quota / private-mode failure paths
• yarn build clean

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