fix(viewer): a file with % in its name opens as itself (#2933)

codegraph ui couldn't show a file whose name contains a %: 100%.ts was
refused as an unusable path, and a%20b.ts was served as changed on disk
with no lines. resolveProjectFile, the chokepoint for every file the
viewer reads, percent-decoded its path. But that path is either one the
index stored or a query value the URL parser had already decoded once,
so this was a second decode. It threw on 100%.ts and turned a%20b.ts into
a b.ts, while resolveRequestedFile looked the file up in the index by the
name as given: the bytes of a b.ts were hashed against a%20b.ts's record.

The chokepoint no longer decodes. It refuses raw control bytes, and on
Windows a backslash, then checks containment as before. Raw request
paths (the viewer's own assets) are still decoded where they are read.
The File view's /api/file/<path>, whose path the HTTP layer also decodes
once, had the same double decode and is fixed by the same change.
The POSIX half of PR #2009 (backslashes kept in names on POSIX) is left
out, as it needs a wider change and a POSIX run.

Validation on Windows 11, with a fixture holding src/a%20b.ts,
src/a b.ts and src/100%.ts:
- codegraph ui over HTTP, /api/source with the query a browser sends:
  main serves a%20b.ts as drift=true with no lines and answers 403 for
  100%.ts; the fix serves all three files as themselves, drift=false.
- Tests: 2 new tests (the chokepoint, and /api/source plus
  /api/file/<path> over HTTP) fail on main's source and pass with the
  fix. Two
  existing cases that fed an encoded ..%2f and %00 straight to the
  chokepoint now go through the URL parser's single decode, as requests
  do, and are still refused.

Reported in PR #2009 by @shrankr; this reimplements the viewer half of
the fix independently.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/viewer-launch-changelog.md

@@ -219,6 +219,8 @@ These describe `codegraph ui` and its screens. They were taken out of `## [Unrel
 
 - **`codegraph ui` won't open a sensitive folder by another name.** A link or junction to a folder like `~/.ssh` or `~/.config`, a differently capitalized name on Windows and macOS, or a `\\?\` spelling on Windows got past the check that keeps such a folder from being opened, and the viewer then showed the files inside. They are now refused like the folder itself. Thanks @mixxer for finding it.
 
+- **A file with `%` in its name opens as itself.** The viewer decoded a file's name a second time before reading it, so `100%.ts` was refused, and `a%20b.ts` was shown as changed on disk, with no lines, because `a b.ts` was read in its place. Every file now opens as the file it names. Thanks @shrankr.
+
 - **Saved trails record the right author when one process serves several projects.** An app embedding the viewer for more than one project wrote the first project's git user name on every trail it saved; each project now uses its own. Thanks @danusha2345.
 
 - **`codegraph ui` on Windows moves on from a port another program holds.** Windows refuses a port that another program keeps for itself, or one inside a range it reserves, with a different error than a port that is simply in use, so the viewer gave up at the first such port instead of trying the next one, and blamed ports below 1024 needing elevated privileges — a rule Windows doesn't have. It now moves on to the next free port, and when none is left, or the port you pinned with `--port` is refused, it says what Windows did and how to list the reserved ranges. Thanks @ijbranch for the report and @sx4im. (#2299)