feat: add -account and -list-accounts flags for Amazon

[eshaffer321]

Summary

AMAZON_ACCOUNT_NAME worked but was poor UX for two real usage patterns:

• Manual one-off runs: you have to remember the env var and the exact profile name, usually by digging through shell history — unlike ./itemize walmart/./itemize costco, which need no extra state.
• Cron jobs: the account name is hidden in an env var assignment rather than visible in the command itself.

Changes

• -account <name> flag — overrides AMAZON_ACCOUNT_NAME for a single run (amazon only)
• -list-accounts flag — scans the browser data directory (~/.itemize/amazon by default) and lists saved profiles, so you don't have to remember exact names:

  $ ./itemize amazon -list-accounts
  Saved Amazon accounts:
    amazon-erick
    amazon-sarah
    amazon-wife
    default
  
  Use with: itemize amazon -account <name>
  

• The resolved account now prints in the run header (Provider: Amazon | Lookback: 14 days | Account: amazon-wife) so it's never ambiguous which profile a run used
• AMAZON_ACCOUNT_NAME still works unchanged as the fallback default — existing cron jobs need no changes
• README documents the new flags under a "Multiple Amazon accounts" section

Files changed

• internal/cli/flags.go — new flags
• internal/cli/providers.go — NewAmazonProvider takes an account override; new ListAmazonAccounts
• internal/cli/providers_test.go — new tests for ListAmazonAccounts
• internal/cli/output.go — PrintConfiguration shows the resolved account
• internal/cli/serve.go — updated call site for the new NewAmazonProvider signature (API server path unaffected, always uses config default)
• cmd/itemize/main.go — wires -list-accounts short-circuit and passes -account through
• README.md — usage docs

Verification

• go build ./..., go vet ./..., go test ./... -race all pass
• Manually ran ./itemize amazon -list-accounts against real local profiles, confirmed correct output
• Manually confirmed -list-accounts on walmart/costco exits with a clear error instead of silently doing nothing

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.95%. Comparing base (ccb6948) to head (9b21c5e).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #56   +/-   ##
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  Coverage   59.95%   59.95%           
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  Files          47       47           
  Lines        5262     5262           
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  Hits         3155     3155           
  Misses       1920     1920           
  Partials      187      187           

Flag	Coverage Δ
unittests	59.95% <ø> (ø)

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