feat(files): download prefill + datamap row affordances

[Nic-dorman]

Summary

Three small file-ops UX fixes, scoped per commit.

• Download-by-Address filename prefill + extension preserve. When the pasted address matches an entry in upload history, the filename field auto-fills with the original name. On submit, if the user dropped the extension while editing, it's silently appended back from the history match. Fixes the long-standing "type 'screenshot', save 'screenshot' that the OS can't open" foot-gun.
• Datamap row Copy Path icon + clearer click affordance. The datamap filename in the uploads table was already click-to-reveal, but visually subtle. Now it picks up a hover underline so the affordance reads as clickable, and a Copy Path icon sits alongside it for users who want the path on the clipboard rather than opened in Explorer.
• Swap Date and Address columns in the uploads table. Address now sits adjacent to the row-action column (Retry / ✕ on hover), grouping the interactive controls on the right edge. Date moves inward as a static metadata cell.

No backend changes. No new Tauri commands. Downloads table is unchanged.

Test plan

• Filename prefill — open Download by Address, paste an address from an existing upload → filename field populates with the original name
• Extension preserved — edit the prefilled filename to drop the extension (e.g. remove .png), click Download → saved file still ends with .png
• No-match passthrough — paste an address NOT in history → filename stays blank, user types freely, no extension append on submit
• Datamap row Copy — upload a small private file, hover the filename in the datamap cell → it underlines + turns blue; click the Copy icon → clipboard contains the absolute path; toast confirms
• Datamap row Reveal — click the filename text → Finder / Explorer opens focused on the datamap file
• Public-row regression — upload a public file → row still shows the "Public" badge with truncated address, click still copies the network address
• Address-only-row regression — a row with file.address but no datamap (legacy / public completion) → still shows truncated address, click still copies
• Hover actions — hover any settled upload row → Retry (if applicable) and ✕ buttons appear on the right edge
• Column order — uploads table reads Name | Status | Size | Cost | Date | Address | (actions on hover)
• Cross-platform — at minimum Windows + macOS

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