[buggedcom]

Closes #70

Problem

The date-window dialog (opened from the comparison-tab rail +) had two user-facing bugs and a cramped layout:

1. Name silently wiped — typing a name then nudging the dates or dragging the range timeline erased it. The name field was bound .value=${this.name} with no change handler, so every parent re-render re-applied the stale prop.
2. "Create date window" appeared to do nothing — with an empty name or an invalid range the panel returned early with no feedback, and nobody called shake().

Solution

Ownership of name state and validation moves into the date-window-dialog molecule (single seam), per the spec's agreed design.

• Name is dialog-internal. The name prop now only seeds internal _nameValue on the open false→true transition (via willUpdate/_wasOpen); the user's typed value is authoritative thereafter, so parent re-renders no longer clobber it.
• Self-validation + shake on submit. Submit validates a non-empty trimmed name and a parseable start strictly before end. Invalid submits shake, surface a localized inline error keyed to the offending field (and focus it for screen-reader announcement), and do not emit dp-window-submit; the dialog stays open. The error clears once the field is corrected. Only valid submits emit { name, start, end }.
• Visual redesign of date-window-dialog.styles.ts — grouped name / date-range (start + end + timeline) / shortcuts / actions sections, timeline breathing room, emphasised primary button with a separated destructive Delete, and preserved @media (max-width: 720px) stacking.
• Panel simplification — _createDateWindowFromDialog's guard is now a defensive assertion; the panel keeps owning the draft range, start/end, bounds, zoom and snapping, plus close/delete/shortcut/date-change handling. Event shapes are unchanged.
• i18n — both new error strings added across de/es/fi/fr/pt/zh-hans. New context.md documents the ownership model.

Tests

• date-window-dialog.spec.ts (molecule seam): name persistence across stale re-renders, invalid submit (empty name / bad range) → no emit + shake + inline error, error clears on correction, edit-mode prefill preserved, happy-path valid submit.
• Panel happy-path coverage (datapoints-mounted-dialogs.spec.ts) unchanged — validity is now guaranteed before the event fires.
• Stories: EmitsSubmit uses a valid named window; added InvalidSubmitShowsError; added a setFrontendLocale locale loader.

Verified locally: tsc, eslint, prettier, the dialog + translation-coverage + panel-dialog unit specs, and all Storybook interaction tests pass.

⚠️ The visual redesign will produce Chromatic diffs that need human baseline approval.

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