feat: Settings page for the log path and poll interval

[kycasdzxc]

Adds a project-level PersistentStateComponent (logPath, pollSeconds) and a
Configurable under Settings → Tools → Stacktale — a text field for the path,
a spinner for the interval.

One thing that changed since the issue was written: the poll has moved out of
StacktalePanel into StacktaleReportService, which the tool window and the
status-bar widget now share. So that is where both settings are read, but the
note on the issue still applies — poll() reads the interval when it re-arms
rather than capturing it once, so a changed interval applies on the next tick
instead of only after the project is reopened.

Defaults reproduce today's behaviour exactly: an empty path keeps the
errors-ai.log auto-detect, and the interval starts at the 3s the plugin has
always used. An existing project notices nothing until a setting changes.

The defaults, bounds, and path resolution live in a plain-Java StSettings in
:core so the suite covers them, per CONTRIBUTING — a stored 0 clamps to 1s
rather than reaching the poll as a busy loop, a relative path resolves against
the project root, and text the filesystem cannot represent is rejected instead
of thrown. StacktaleSettings keeps the persisted state and applies those
rules; the settings page reads its spinner bounds from the same place.

One judgement call worth flagging: when a path is set but no file is there,
the auto-detect does not run as a fallback. Falling back would quietly show
reports from a file the user did not pick, so the tool window says the
configured path has no file instead. Happy to invert it if you meant the other
way round.

Verification

• ./gradlew test — 15 green (7 parser + 8 new settings tests)
• ./gradlew build verifyPluginProjectConfiguration verifyPluginStructure —
  green against IntelliJ IDEA Community 2024.3.5
• ./gradlew runIde — the sandbox IDE loads the plugin with no errors in the log
• Also verified headlessly with a throwaway BasePlatformTestCase (not included
  in this PR, since :plugin's test task is disabled by design): a configured
  path is the file the poll reads, clearing it returns to auto-detect, a missing
  configured path yields no reports rather than a silent fallback, and the
  settings page round-trips both values.

Closes #5

[GabrielBBaldez]

This is a careful piece of work, and thank you for coming back to it — the check-in was meant as an offer, not a nudge, and you more than answered it.

Verified rather than taken on trust:

• 15 tests, 0 failures — 7 parser plus your 8, matching what you reported.
• POLL_MILLIS = 3000 on main and DEFAULT_POLL_SECONDS = 3 here, so an existing project genuinely notices nothing.
• The interval really is read on re-arm rather than captured, so a change lands on the next tick.
• StacktaleSettings registers through @Service + @State rather than plugin.xml, which is why the XML only gained the configurable — correct, and easy to get wrong in the other direction.

Your judgement call is the right one, and I would not invert it. A configured path that has no file should say so rather than fall back to auto-detect: falling back would show reports from a file the user did not choose, and being quietly wrong about which file you are reading is worse than being visibly empty. Same reasoning applies to the && autoDetecting you added to the dumb-mode guard — indexing only matters when detection is doing the work — and I appreciated finding the comment already there explaining it.

Putting the defaults, clamping, and path resolution in :core as plain Java is what made all of the above checkable from a test rather than from an IDE, and it is the part I would point at if someone asked how to add a setting here later.

Thank you also for saying plainly that the poll had moved since the issue was written, instead of quietly working around it. Merging.

[github-actions]

Merged — thanks @kycasdzxc, that's your first one here.

If the project turned out to be useful to you, a ⭐ genuinely helps: stacktale is
new, and stars are most of what decides whether anyone else finds it.

Either way you're welcome back — the good first issue
list is kept honest, and each one names the files to touch and how to verify.