[feat] Right-click string utilities on selected text (encode/decode/stringify/destringify)

[mertbekci]

Split out from #81.

Background

The CellContextMenu component already exists on URL bar / param / header inputs (right-click → copy / paste / clear). Extend it with text-transform utilities that act on the current selection when one is present, otherwise on the full cell value.

Proposed change

Add a Transform selection submenu (or four top-level items, TBD by feel during implementation):

• URL-encode — encodeURIComponent semantics
• URL-decode — decodeURIComponent with try/catch (malformed URLs leave selection untouched and toast an error)
• Base64 encode — UTF-8 safe, no padding mangling
• Base64 decode — symmetric inverse
• Stringify — escape for embedding in a JSON string literal: "foo" → \\\"foo\\\"
• Destringify — symmetric inverse: \\\"foo\\\" → "foo"

Where it should be available

• URL bar
• Param key + value cells
• Header key + value cells
• Body editor (raw / json / xml / html / javascript modes — CodeMirror selection)
• Form-urlencoded fields
• Multipart text parts

GraphQL editor follows the same CodeMirror-selection rules.

Acceptance criteria

• Transforms apply to the current selection (or whole field if no selection).
• Each transform is reversible: encoding then decoding the same string returns the original.
• Decode-of-malformed input shows an error toast, leaves the field untouched.
• Available in URL bar, param/header cells, and the body CodeMirror editor.
• Unit tests for each pair (round-trip + malformed input).
• Keyboard shortcuts not assigned (avoid clashing); discoverable via right-click only.

Out of scope

• User-defined transforms (plugin system). See #C-issue for the parking lot.
• Format converters (JSON ↔ YAML, XML ↔ JSON). Track separately if requested.

Parent: #81

[mertbekci]

Shipped in v0.6.1 (commit 200174f).

What landed:

• New menu items Stringify and Destringify — escape (and unescape) for embedding inside a JSON string literal. Stringifying say "hi" produces say \\"hi\\"; destringifying returns the original.
• All six transforms (URL encode/decode, Base64 encode/decode, Stringify, Destringify) are now selection-aware. When the focused input/textarea has a non-empty range, only that range is transformed and the rest of the field value is preserved. With no selection, they apply to the whole field.
• After a selection-mode transform, the freshly transformed range is re-selected so a user can round-trip with another menu pick.
• Copy / paste also honour the selection: copy emits the selected substring; paste replaces only the selected range.
• Pure helpers extracted to utils/string-transforms.ts with 12 unit tests covering ASCII, multi-byte unicode, malformed input, and round-trip stability for every transform pair.

Out of scope here: body editor (CodeMirror raw mode). Its selection model is different — separate issue if requested.