docs: rebuild the README Screen Map from the code (closes #164)

The Screen Map had drifted from src/popup/ in both directions. Every entry
was re-verified against the view modules, the view registry in
src/popup/views/helpers.js, the navigation stack in src/popup/index.js and
the markup in src/popup/index.html, and rewritten to match.

Screens added: ExportPrivKey, DeleteWallet, SettingsAddToken.

Flow descriptions corrected: AddWallet is one screen with three tabs, not
an AddWallet screen plus an ImportKey screen reached from a link that does
not exist; ConfirmTx takes an inline password field, not a modal; the
SiteApproval and TxApproval popup mechanisms were swapped relative to
src/background/index.js; a Home transaction row opens TransactionDetail,
not AddressDetail. Back transitions now say "previous screen" with the
actual origins, since every Back button pops the navigation stack.

Also: the xprv wallet type added to the Data Model, the Screen Map and
Supported Functionality; Settings now lists every element it has; the
"Gas estimation and fee display before confirming" TODO checkbox ticked,
as confirmTx.js implements it; and the Clickable Affordance example no
longer cites a UI string that does not exist.

Documentation only. Verified with make check (7 suites, 143 tests, prettier
clean) and with script/cibuild, whose containerized make check ran
uncached.

TODO.md

@@ -44,6 +44,9 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-11: README Screen Map rebuilt from the code — every screen, element
+  and transition re-verified against `src/popup/`
+  ([#164](https://git.eeqj.de/sneak/AutistMask/issues/164)).
 - 2026-08-11: `TODO.md` Workflow rewritten to the branch-and-PR-per-issue model
   on `next`, with Status and Next Step refreshed
   ([#191](https://git.eeqj.de/sneak/AutistMask/issues/191)).