docs: code comments name the headings README has now (closes #724)

The README rewrite renamed or merged several headings that comments still
cited. Each comment now names the current heading. Line number citations of
README become headings too, and one test name drops its line numbers.

The e2e workflow's citations of "In CI" are dropped, because no section has what
they pointed at. Its comment also corrects what it says about required jobs and
open reports.

The comment on withLock() no longer states a Firefox private window exception,
because the manifest keeps the extension out of those windows.

Two comments also take corrections from reviews: the cases with no total in
DeleteAddress's warning, and the screens that say a swap's amount could not be
decoded.

Model: opus-5-5

src/background/state.js

@@ -18,11 +18,11 @@
 //   updateState(fn)   read fresh, apply fn to that fresh record, write it
 //                     back — all holding the lock every write of the stored
 //                     record takes, so no other write lands in between, by
-//                     the background or by any page (README, "Stored state
-//                     and its version"). The read is one storage round trip
-//                     ahead of the write rather than a page lifetime ahead of
-//                     it (which is what made the popup's saveState() need a
-//                     per-field merge against a baseline at all).
+//                     the background or by any page (README, "Stored data").
+//                     The read is one storage round trip ahead of the write
+//                     rather than a page lifetime ahead of it (which is what
+//                     made the popup's saveState() need a per-field merge
+//                     against a baseline at all).
 //
 // A handler that must both read and write therefore does its network work
 // against a snapshot it owns, and applies the RESULT inside updateState().
@@ -72,7 +72,7 @@ async function updateStateOnce(mutate) {
 // src/shared/browserApi.js) is held from the read to the write: two
 // background turns, or a turn and a page's save, never interleave
 // (https://git.eeqj.de/sneak/AutistMask/issues/641). Every page's save waits
-// while it is held (README, "Stored state and its version").
+// while it is held (README, "Stored data").
 //
 // `mutate` must also not call updateState() itself, directly or through
 // anything it awaits: the inner turn waits for the lock the outer one holds,