Standalone Decoder: URL/Base64/Hex/HTML encode & decode

First of the remaining "worth considering" items. A self-contained
tool ('d' from the history list, not seeded from any entry - this is
for arbitrary snippets, pasted tokens, encoded parameter values) with
a vi-modal input pane and a live output pane that updates on every
keystroke and every transform switch (tab/shift+tab cycles through the
8 transforms).

decoder.go is pure logic, deliberately kept separate from the TUI
wiring so it's directly testable: urlEncodeAll implements strict RFC
3986 percent-encoding (space -> %20) rather than using Go's
url.QueryEscape, whose form-encoding behavior (space -> '+') isn't what
"URL encode" means to a pentester reaching for this tool. Base64 decode
tries standard/URL-safe/padded/unpadded encodings in turn rather than
requiring the user to know which one they're looking at - real pasted
data is as likely to be one as the other. Decode failures return a
visible "(error: ...)" placeholder rather than blanking the output, so
a bad guess at the transform is obviously wrong rather than looking
like nothing happened.

decoder_test.go covers each transform directly, three "this input isn't
valid for this transform" error cases, and a round-trip matrix (all 4
encode/decode pairs against 5 inputs chosen to be awkward for at least
one encoding - spaces, slashes, HTML-special characters, empty string,
embedded newlines) confirming encode-then-decode always recovers the
original.

Single-transform only, not chained/pipelined like Burp's Decoder - v1
scope, tracked in PLAN.md.

Verified live: typed text and watched the output pane update in real
time; confirmed URL-encoding, then cycled to Base64 via tab and watched
it re-encode the same input live; confirmed the active-transform
highlighting via raw ANSI codes in the captured pane; fed invalid input
to Base64 decode and confirmed the error placeholder renders instead of
silently showing stale output; confirmed esc correctly backs out to the
history list.

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