Timezone-aware local timestamps in all export filenames

[NickMonrad]

Problem

All export filenames that include a date currently use new Date().toISOString().slice(0, 10) which always produces a UTC date. This means users in UTC+10/+11 (e.g. AEST/AEDT) can receive files stamped with yesterday's date when exporting in the evening. Additionally, no exports include a time component, making it hard to distinguish multiple exports on the same day.

Proposed Solution

1. Pass the user's local timezone from the browser to the server on export requests (via a query param, e.g. ?tz=Australia/Sydney)
2. Format all export timestamps as YYYY-MM-DD HH-mm in the user's local timezone (colons replaced with hyphens for filesystem safety)
3. Apply universally across all exports that currently have a date, and add timestamps to exports that currently have none

Affected Files & Locations

Server-side (date via query param ?tz=)

File	Route	Current format
server/src/routes/csv.ts:160	GET .../backlog/export-csv	YYYY-MM-DD via toISOString()
server/src/routes/timeline.ts:1214	GET .../timeline/export/csv	YYYY-MM-DD via toISOString()

Client-side (can use new Intl.DateTimeFormat directly)

File	Export type	Current format
client/src/pages/EffortReviewPage.tsx:228	Effort Summary/Detail CSV	YYYY-MM-DD via toISOString()
client/src/pages/TimelinePage.tsx:296	Timeline CSV	YYYY-MM-DD via toISOString()
client/src/pages/TimelinePage.tsx:351	Gantt PNG	YYYY-MM-DD via toISOString()
client/src/pages/ResourceProfilePage.tsx:603	Resource Profile CSV	No timestamp
client/src/pages/ResourceProfilePage.tsx:616	Project Export ZIP	No timestamp

Server-side document download

File	Route	Current format
server/src/routes/documents.ts:87	GET .../documents/:docId/download	No timestamp (label only)

Template exports (add timestamp for consistency)

File	Export type	Current format
server/src/routes/templates.ts:91	All templates CSV	Static templates.csv
server/src/routes/templates.ts:258	Single template CSV	Static {slug}.csv

Acceptance Criteria

• A shared utility formatExportTimestamp(tz?: string): string returns YYYY-MM-DD HH-mm in the given IANA timezone (falls back to UTC if invalid/missing)
• All server export routes accept an optional ?tz= query param and use it for the timestamp
• All client export call sites pass tz= (Intl.DateTimeFormat().resolvedOptions().timeZone) as a query param
• Client-only exports (Effort, Timeline CSV, Gantt PNG, Resource Profile, ZIP) use the same local timestamp format via a shared client utility
• Document downloads include a local timestamp: {label} - YYYY-MM-DD HH-mm.{ext}
• Template exports include a local timestamp: templates - YYYY-MM-DD HH-mm.csv / {slug} - YYYY-MM-DD HH-mm.csv
• No export filename still uses raw toISOString() for its user-visible date/time