[amar-python]

Part of #118. It doesn't close it: the cause of the 27 Oct silent API stop still needs evidence from the dev machine (see below).

What changes

Part	Change
Browser message	A new callApi() in session.ts turns a network failure into "Can't reach the API at http://localhost:8000. Is it running?" instead of the browser's bare "Failed to fetch". Cancelled requests (AbortError) pass through unchanged. Sign-in, sign-out, change password and every apiFetch call use it.
start-api.ps1	Before starting, it checks port 8000. If something already listens there, it prints the PID, process name, start time and the taskkill line, then exits 1. On 27 Oct an API left over from 02:12 kept the port, and a new API printed "running" but never served a request.
Window titles	start-api.ps1 and start-frontend.ps1 set their window titles to "API (port 8000)" and "Frontend (port 8080)", as the product owner asked on 27 Oct.
QUICKSTART.md	Two troubleshooting rows for the new messages. The stale "alembic.ini not found … no Alembic in this repo" row is removed, because alembic.ini has existed since #84.

Checks

• vitest: 134 passed (it was 131). The new tests cover: sign-in while the API is down; another call while it's down (the session is kept); a cancelled request isn't reported as the API being down.
• Mutation checks:
  • removing the network-error mapping makes 2 tests fail;
  • mapping every error, including aborts, makes 1 test fail.
• tsc exit 0; eslint clean on session.ts.
• PowerShell: both scripts parse with 0 errors (pwsh Parser.ParseFile) and stay ASCII-only (BUG-021). The port check itself (Get-NetTCPConnection) is Windows-only, so it couldn't be run here. It needs the dev-machine check below.
• make lint exit 0. No Python changed.

Still open on #118 (needs the dev machine)

1. Port check: with the API running, start a second .\scripts\start-api.ps1. It should print "Port 8000 is already in use by PID …" and exit.
2. Silent stop: the cause of the morning stop hasn't been identified. Possible lead: --reload reacting to OneDrive syncs.

Done when

• This PR is merged with CI green.
• The port-check screenshot is attached to Frontend shows 'Failed to fetch' when the API is down; the local API stopped silently once #118.
• Frontend shows 'Failed to fetch' when the API is down; the local API stopped silently once #118 stays open until the cause of the stop is identified or ruled out.

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