Add the Sell tab: post a listing from one screen

[AndrewG828]

Stacked PR 9 of 15. Based on #118 — merge in order.
#111 → #112 → #113 → #114 → #115 → #116 → #117 → #118 → #119 → #120 → #121 → #122 → #123 → #124 → #125

Replaces the three-step push flow (add button → images → details) with a single
form: photos, title and price, category, condition, description, publish.
Nothing routes to it yet — the tab bar rework wires it up.

The photo well is a carousel rather than a strip, so each image is reviewed at
the size it will be posted at, with delete and add controls on the image itself
and arrows only where there is somewhere to go.

Categories come from Constants.filters instead of a hand-written list. The
first cut hardcoded seven, which already disagreed with the taxonomy — it was
missing Household. Only the "Handmade" → "Homemade" display name is
special-cased here.

New listings default to "Gently Used" rather than "Never Used": the previous
default made the strongest claim on the seller's behalf, which is the wrong way
round for a form that people will submit without reading.

Left out of the port: a clothing-only "Additional Details" block whose call site
was commented out and whose fields did nothing.

Worth flagging for a follow-up — NewListingViewModel.createNewListing() calls
clear() on failure as well as success, so a network error silently empties the
form. That was survivable when the form was a pushed screen you were about to
leave; on a tab root that keeps you where you are, it reads as the app throwing
your listing away.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_017R8mua9xepzF3mnRETtd6W

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