feat: support custom delimiters in the serve web workbench

[machado144]

Summary

• The serve web workbench could only ever display the active [[ ]] delimiter pair (set at startup via --startDelim/--endDelim); changing it meant restarting the server. This adds an editable pair next to the directory path — type a new pair and click Set to validate, apply, and rescan instantly, no restart.
• The new pair is stored on the running Server and read by settings(), so it applies uniformly to Local, Clone, and Generate — the same single source of truth the CLI already uses.
• Zero new matching/replacement logic: this reuses the exact same workflow.Engine / services.Replacer functions the CLI's template, clone, and generate commands already exercise (regexp.QuoteMeta-escaped regex for apply, strings.Index literal scan for preview — both already covered by services/replacer_test.go).
• Adds ValidateDelimiters, which closes a real bug found while building this: the literal scan (services/replacer.go) finds delimiters with strings.Index, which returns 0 without consuming input for an empty needle — an empty/empty pair would spin forever on any non-empty file, hanging the request goroutine. Empty, equal, and mutually-containing delimiter pairs are now rejected up front (400 + clear message), and a rejected change leaves the previous pair active.
• Also fixes a lock-discipline gap: settings() and handleIndex now read startDelim/endDelim under Server.mu, since they're mutable at runtime now instead of fixed at construction (verified race-free with go test -race).
• Docs updated: README.md, docs/user/README.md, docs/AI/README.md, EXAMPLES.md, COMMANDS.md.
• Includes a small, separate first commit swapping the placeholder banner for the new logo and adding a recorded demo GIF of the workbench.

Test plan

• go build ./..., go vet ./..., gofmt -l clean on touched files
• go test ./... -race — full suite green, including the new tests, no data races
• New unit tests for ValidateDelimiters (table-driven): valid custom/default/unicode/regex-metachar/equal-length-non-overlapping pairs, plus empty/whitespace-only/equal/mutually-containing rejections
• Regression test (TestSetDelimitersRejectsEmptyWithoutHanging) that guards the empty-pair hang bug with a 2s timeout, so it fails loudly instead of hanging CI if the validation is ever removed
• HTTP-level feature test proving a full before/after: default pair only sees one placeholder, switching delimiters rescans and finds the other, then /api/apply writes the file using the new pair while an unrelated file using the old pair is untouched
• HTTP-level test that a rejected /api/delimiters call returns 400 with a message and leaves server state (and subsequent scans) unchanged
• Method-not-allowed and malformed-JSON edge cases on the new endpoint
• Concurrency test hammering SetDelimiters/Scan from goroutines simultaneously (clean under -race)
• Manually exercised in a real browser end-to-end (screenshots below): scanned a dir with default delimiters, switched to <%/%> live, watched the placeholder list update and old matches disappear, applied successfully, then confirmed the validation error path shows a clear message and leaves state untouched

Before → after (live in browser)

Default [[ ]] scan	After setting <% %>	Rejected invalid pair
finds NAME only	finds GREETING only, banner confirms rescan	red banner: "start and end delimiters must be different", state unchanged

🤖 Generated with Claude Code

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