Add UI for setting repeater identity keys

This adds a simple UI in repeater management settings for remotely
setting a repeater's private key. A key can be pasted from an external
key generator, or a random one can be generated here, optionally with a
specified prefix.

If a prefix is specified, searching is done in 30ms batches in order
to keep the UI responsive and show the progress indicator animation
smoothly. Searches can be interrupted. An estimation of search length is
given as expected number of generated key pairs. (A future improvement
would be to observe keys per second performance and give time estimates
based on that.)

When it generates a new private key on your behalf, it can tell you
what the corresponding public key will be. Unfortunately, if you paste
one in from another source, it can't compute the corresponding public
key. This is because the standard Dart crypto library doesn't provide
that capability, nor does it give access to the elliptic curve functions
it implements so we could do it ourselves. Our only recourse would be to
copy the source for all those things, and I didn't think it was worth
it.

This will not check the validity of a pasted key, but the remote
repeater will. It does limit it to the correct length and hexadecimal
charset.

Prefixes are limited to 4 characters. There's no technical reason for
this, but users are unlikely to want more, and I think it would just be
a bad experience to allow more and spin for a very long time. If someone
really wants a longer one, they can use an external generator like they
do now.

Possible TODO: rewrite background processing using isolates.

Closes #142.

lib/l10n/app_en.arb

@@ -1409,6 +1409,26 @@
   "repeater_advancedSettingsSubtitle": "Tuning knobs for experienced operators",
   "repeater_pathHashMode": "Path hash mode",
   "repeater_pathHashModeHelper": "Bytes used to encode this repeater's ID in flood path/loop-detect tags. 0=1 byte (256 IDs, up to 64 hops), 1=2 bytes (65K IDs, up to 32 hops), 2=3 bytes (16M IDs, up to 21 hops). v1.13 and older firmware drops multi-byte paths — only raise once your network is on v1.14+.",
+  "repeater_keySettings": "Change Identity Keys",
+  "repeater_keySettingsSubtitle": "Change the public/private keypair",
+  "repeater_prvKey": "Private key",
+  "repeater_prvKeyHelper": "A new private key for the repeater, a 128-character hex string.",
+  "repeater_generatePrvKey": "Generate a random keypair",
+  "repeater_stopGeneratingPrvKey": "Interrupt search for keypair",
+  "repeater_pubKey": "Public key",
+  "repeater_pubKeyHelper": "This is the public key that goes with the generated private key. You can't set this directly.",
+  "repeater_pubKeyPrefix": "Desired prefix",
+  "repeater_pubKeyPrefixHelper": "Find a public key that starts with these hex digits. Expected tries needed: {tries}.",
+  "@repeater_pubKeyPrefixHelper": {
+    "placeholders": {
+      "tries": {
+        "type": "int"
+      }
+    }
+  },
+
+
+
   "repeater_txDelay": "Flood TX delay",
   "repeater_txDelayHelper": "Retransmit spacing for flood traffic, as a multiplier of the packet's airtime (0-2, default 0.5). Higher = fewer collisions but slower delivery.",
   "repeater_directTxDelay": "Direct TX delay",
@@ -1589,7 +1609,7 @@
   "repeater_cliHelpPowersavingOnOff": "Enables or disables powersaving mode (where supported).",
   "repeater_cliHelpErase": "(Serial only) Formats the device file system. Wipes all settings and contacts.",
   "repeater_cliHelpSetDutyCycle": "Sets the maximum allowed transmit duty cycle as a percentage (1-100). Internally adjusts the airtime factor.",
-  "repeater_cliHelpSetPrvKey": "(Serial only) Replaces the device identity private key. Reboot required to apply. Generates a new public key.",
+  "repeater_cliHelpSetPrvKey": "Replaces the device identity private key. Reboot required to apply. Generates a new public key.",
   "repeater_cliHelpSetRadioRxGain": "(SX126x only) Toggles boosted RX gain for improved sensitivity at higher current draw.",
   "repeater_cliHelpSetOwnerInfo": "Sets the owner contact info string included in adverts. Use '|' for newlines.",
   "repeater_cliHelpSetPathHashMode": "Sets the path-hash mode. 0 = legacy, 1 = standard, 2 = strict. Affects how routing paths are matched.",